In a general semiconductor device, an insulator is used for element isolation or as a dielectric.
Herein, referring to FIG. 3, the structure of a general semiconductor device will be described.
Source/drain diffusion layers 20 are formed in an element forming region of a silicon substrate 1. An element isolation region (STI portion) composed of a silicon oxide film (SiO2 film) 12 and an insulating film (SiO2 film) 13 is formed around the element forming region. A gate electrode 15 is formed on the element forming region through a gate insulating film 14 and an interlayer insulating film 18 is formed so as to cover the element forming region and the element isolation region.
Sidewalls (Si3N4 film) 16 are formed on the side walls of the gate electrode 15 and a SiO2 film 17 is formed so as to cover the gate insulating film 14 and the gate electrode 15. Further, the SiO2 film 17 is also formed between the silicon substrate 1 and the interlayer insulating film 18. Metal layers (plugs) 19 are connected to the source/drain diffusion layers 20 and the gate electrode 15, respectively.
Conventionally, the insulator is basically formed of a silicon oxide (relative dielectric constant: 3.9).
Currently, in uses other than a function of inducing electric charge, a reduction in dielectric constant is required for suppressing signal delay and the dielectric constant is reduced by adding fluorine to a silicon oxide or providing an air gap.
Although use is made of a silicon oxide film (SiO2 film) for directly isolating a transistor or isolation between transistors by the use of an element isolation region such as LOCOS or STI (Shallow Trench Isolation) (see FIG. 3), it will be necessary in future to achieve a reduction in dielectric constant also for this portion.
Currently, a material used for isolation between transistors is formed on the basis of silicon such as a silicon oxide film (SiO2, ∈=3.9) or a silicon nitride film (Si3N4, ∈=7.8). However, there is a problem that the relative dielectric constant of each of them is about 3.0 even by adding F or C and it is difficult to achieve a reduction in dielectric constant if the silicon nitride film is combined.